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Effects of plasma modification on adhesion properties of wood
International Journal of Adhesion & Adhesives 32 (2012) 70–75
Contents lists available at SciVerse ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effects of plasma modification on adhesion properties of wood Menandro N. Acda a,n, Edgar E. Devera a, Rico J. Cabangon b, Henry J. Ramos c a b c
Department of Forest Products and Paper Science, University of the Philippines Los Banos College, Laguna 4031, Phillipines Forest Products Research and Development Institute, Department of Science and Technology, Laguna 4031, Phillipines National Institute of Physics, University of the Philippines Diliman, Diliman, Quezon City 1101, Phillipines
a r t i c l e i n f o
a b s t r a c t
Article history: Accepted 10 October 2011 Available online 26 October 2011
The study investigated the use of dielectric barrier discharge for surface modification to improve adhesion properties of Shorea contorta (white lauan), Gmelina arborea (yemane) and Acacia mangium. Wood specimens were exposed to oxygen plasma at intensity levels ranging from 5.8 to 46.5 kW min/ m2 to improve adhesion of phenol formaldehyde, urea formaldehyde resins and polyurethane coating. Work of adhesion was calculated based on contact angle measurements to determine thermodynamic changes on plasma modified wood. Surface characteristic was evaluated using atomic force microscopy (AFM). Results of the study showed that plasma modification resulted in significant improvement in work of adhesion for the three wood species investigated. Mechanical tests of plywood and wood laminates using plasma treated S. contorta glued with phenol or urea formaldehyde resins indicated improvement in shear strength of adhesive joints. No improvement or decrease in shear strengths were observed for plasma treated G. arborea and A. mangium. Pull off strengths of polyurethane coating on plasma treated S. contorta and A. mangium specimens showed slight improvement in strength of coated film. Effects of plasma treatment on adhesion properties of wood appear to be species specific and vary with process parameters. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Plasma Surface modification Wood Atomic force microscopy
1. Introduction In recent years, cold plasmas have been used to modify surface properties of wood and paper to improve adhesion of glues or enhance deposition of thin layer of polymers to the surface of metals, glasses and plastics [1–3]. Cold plasma is a mixture excited particles (ions, electrons, atoms, radicals, etc.) with very low degrees of ionization and little penetrating energy [4]. However, plasma particles have sufficient energy levels to break chemical bonds on the surface of organic and inorganic substrates [5,6]. The broken chemical bonds are thermodynamically unstable and combine readily with gas fragment to molecularly re-engineer the surface of the material [7]. The modification (surface cleaning, ablation and degradation, cross-linking, surface oxidation and polymerization), however, is limited to a few molecular layers and does not affect bulk properties. The effects may occur concurrently or one may predominate depending on the processing conditions and the design of the reaction chamber. Plasma treatments have been successfully applied in a wide range of surface modification and enhanced chemical vapor deposition [1–3]. This approach is also attracting research interest for surface
n
Corresponding author. Tel.: þ49 536 3432; fax: þ 49 536 3206. E-mail address: [email protected] (M.N. Acda).
0143-7496/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.10.003
modification to improve adhesion and performance of wood coatings. Studies showed that plasma can be used to improve the wettability of wood to liquid [6,8–10]. Wettability assists in establishing an extensive and molecular scale contact with the wood surface and critical to the development of strong adhesion at the adhesive–wood interface [11]. Cold plasmas have been used to improve the wetting and adhesion properties of wood [12–17] and wood composites [18–20]. It is believed that plasma modification results in increased surface polarity brought about by oxidation reaction leading to the formation of hydroxyl, carboxyl, aldehyde and other polar functional groups [21–23]. The increased surface polarity improves wettability and penetration behavior (hydrophilicity) of wood to liquid [6,8–10]. Studies on the effects of plasma treatments to improve strength of wood finishes and coating systems, however, are limited. Atmospheric corona discharge, a variation of plasma treatment, has been used to increase the surface energy of wood surfaces by oxidative activation [24]. Improved bonding of a water-based acrylate lacquer was found following corona treatment of wood surface but no improvement when a solvent-based alkyd system was used. Denes and Young [25] coated wood with polydimethylsiloxane (PDMSO) containing various UV stabilizers, absorbers or reflectors, and polymerized the coating using an oxygen plasma. The use of such coatings significantly improved the weathering resistance of the coated
M.N. Acda et al. / International Journal of Adhesion & Adhesives 32 (2012) 70–75
wood. Shear strength of hardwood and softwood wood specimens coated with lacquer adhesive improved with corona treatment while delamination of coatings on wetted bonded samples was reduced significantly [15]. Plasma surface treatment shows tremendous potential for improving adhesion of glues and coatings to wood. Consequently, new material combinations, elimination of primers and cost savings are conceivable benefits [3]. The present paper reports on the use of plasma modification to improve adhesion properties of three species of tropical hardwoods, viz., Shorea contorta Vidal, Gmelina arborea Roxb. and Acacia mangium Willd. commonly used in the Philippines and Southeast Asia as raw material for furniture and wood based panel products.
2. Materials and methods 2.1. Wood specimen
71
calculated using the Dupre equation W A ¼ gS þ gL gSL
ð1Þ
where gS is the surface free energy of solid, gL is the surface tension of the liquid and gSL is the interfacial energy between solid and liquid. For optimum adhesion, WA must be maximized, which occurs when the interfacial tension (gSL) becomes zero [20,27]. The interfacial tension between solid and liquid is given by the geometric mean equation [28] qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi ð2Þ gSL ¼ gS þ gL 2 gDS gDL 2 gPS gPL P D where gD L and gL are the dispersive and polar part of liquid and gS and gPS are dispersive and polar part of solid (i.e. surface free energy of wood species). Since many paints and wood coatings are water based, work of adhesion was determined in terms of dispersive and polar components: P WA ¼ WD A þW A
ð3Þ
and using Eqs. (1)–(3) qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi D D WD and W PA ¼ 2 gPL gPS A ¼ 2 gL gS
ð4Þ
3
Kiln dried S. contorta (white lauan, density 430 kg/m ) lumber obtained from local furniture manufacturer in Cebu province, G. arborea (yemane, density 410 kg/m3) and A. mangium (density 470 kg/m3) from the Forest Products Research Development Institute, Philippines were used in this study. These species are grown intensively in industrial plantations and commonly used throughout the Philippines and Southeast Asia for furniture and panel manufacture. Veneer specimens (2 mm 100 mm 100 mm) and wood blocks (10 mm 50 mm 50 mm) were cut from the heartwood of straight grained and defect-free boards and conditioned for several weeks at 24 1C and 65% relative humidity to bring the moisture content to 8% prior to plasma modification.
2.2. Plasma treatment Plasma surface modification to improve adhesion property of S. contorta, G. arborea and A. mangium wood specimens was performed using a dielectric barrier discharge (DBD) developed at the National Institute of Physics, University of the Philippines, Diliman, Philippines and described in an earlier paper [26]. Briefly, high purity oxygen was admitted between the electrodes at reduced pressure (10 Pa). Mass flow, temperature, pressure and electric potential were closely monitored and recorded using data acquisition instruments. Surfaces of veneers and wood blocks to be coated with glue were exposed to dielectric barrier discharge ranging in intensities from 5.8 to 46.5 kW min/m2 at 3 mm gap between electrodes to determine its effect on adhesion properties. The DBD chamber was thoroughly cleaned with ethanol after each treatment to remove deposits and contaminants from previous treatment. Untreated wood specimens were used as control. Surface characterization of plasma treated and untreated specimens were performed using an atomic force microscope (Solver PRO, NT-MDT, Netherlands) running on a Nova 1.0.26C software. All measurements were performed at atmospheric pressure and room temperature in non contact tapping mode (scan size 1.0– 2.5 mm2, scan rate 0.3–1 Hz with a silicon cantilever tip, 256 pixels by 256 pixels resolution). Topographic and phase contrast images of plasma treated specimens were obtained and compared with untreated controls.
2.3. Work of adhesion The work of adhesion with reference to water was used to assess the effect of plasma treatment on adhesion properties of all three wood species investigated. The work of adhesion (WA) was
The surface free energies (gS ) of S. contorta, G. arborea and A. mangium wood specimens were calculated based on the Owens, Wendt, Rabel and Kaeble method and reported in an earlier paper [26]. Water (gL ) has a known polar and dispersive components (gP ¼51 mN/m, gD ¼21.8 mN/m). 2.4. Adhesive bond strength Three ply plywood and 2-ply wood laminates were constructed using specimens from treatment combination that showed optimum improvement in surface energy and work of adhesion. Plywood was constructed using a commercial hot setting phenol formaldehyde resin (Phenoress, 45% resin solids, RI Chemical Corp., Pasig, Philippines) following gluing and pressing conditions recommended by the manufacturer (glue spread 130 g/m2, specific pressure 10 kg/cm2, press temperature 130 1C, total pres time 3.5 min, assembly time 5 min). Glue was applied on plasma modified surfaces (i.e. underside of face and back veneers; and both sides of core veneer). Wood blocks were laminated (185 g/m2 spread rate, 10 kg/cm2 specific pressure and 24 h press time) using a cold setting urea formaldehyde resin (Consolidated Adhesives, Inc.) commonly used for wooden parquet and flooring in the Philippines. Standard plywood and wood block shear specimens were prepared and used to evaluate the effects of plasma modification on strength properties of adhesive joints in compression and tensile loading. All specimens were conditioned for 6 weeks at 24 1C and 65% relative humidity prior to adhesive bond testing. Bond strength of plywood specimens was measured using the cyclic-boil, hot–cold soaking test of the Philippine National Standard 196 [29]. Dry shear strength of wood block laminates was measured following the American Society for Testing and Material 906 [30]. Testing was carried out using a universal testing machine at crosshead speed of 0.5 mm/min. Shear force required to break the adhesive bond to failure was recorded by data acquisition software. Grain direction was ensured parallel to the shearing stress for all specimen tested. Four replicate specimens were tested for each treatment combination. Data for shear strength and plasma intensity were fitted in a completely randomized design and evaluated using analysis of variance (ANOVA) using Statgraphics Plus for Windows 4 software [31]. Treatment means were separated by Tukey’s Honest Significance Difference (HSD) test (a ¼0.05).
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M.N. Acda et al. / International Journal of Adhesion & Adhesives 32 (2012) 70–75
2.5. Coating and pull-off test Plasma treated and untreated wood specimens were spray coated two times with a two component polyurethane coating system (Topcoat 20-AAA varnish and polyurethane catalyst-AAA, Century Chemical Corporation, Quezon City, Philippines) within 6 h following plasma treatment. Polyurethane coatings are commonly used for wood finishes and parquet floors in the Philippines. A final coating was applied after a light sanding (320 grit). The coated specimens were stored in a conditioning room at 24 1C and 65% relative humidity for 2 weeks prior to mechanical testing. The adhesion of the polyurethane films on both treated and untreated specimens was evaluated using a modified pull-off test following ASTM D 4541 [32]. A universal testing machine (5 kN capacity) fitted with a fabricated jig consisting of a 20-mmdiameter steel dolly and sample holder was used in this study. The dolly was glued on the coated wood surface with an epoxy resin. After 6 h of curing at 24 1C and 65% RH, the perimeters of the glued dollies were carefully cleaned to prevent propagation of failures out of the tested area. Constant pull of the testing machine was applied at 5 mm/min until separation of the dolly from the plasma treated and untreated specimens. The maximum normal pull strength at rupture was recorded using data acquisition software. Pull off strength data was analyzed for each species and compared with untreated controls as described above.
3. Results and discussion 3.1. Effects on work of adhesion Plasma surface modification of S. contorta, G. arborea and A. mangium using dielectric barrier discharge at intensities used in this study resulted in 2–3 times increase in work of adhesion compared with untreated materials (Fig. 1). No apparent surface defects or negative effects on bulk properties were observed following plasma treatment. Improvement in the work of adhesion was predominantly caused by significant increase in the polar component (p40.001) for all three species. Similar increase in work of adhesion were also reported using air or helium DBD treatment of oak, beech, spruce and Oregon pine [33,34]. Since most coating systems for wood are water based, plasma treatment could have potential beneficial effects in improving adhesion properties of S. contorta, G. arborea and A. mangium to polar coating systems. 3.2. Effects of plasma treatment on strength of adhesive bond
Fig. 1. Work of adhesion for untreated and plasma treated S. contorta, G. arborea and A. mangium wood specimens.
2
Plasma intensity at 5.8 kW min/m was the lowest level that resulted in significant improvement in work of adhesion for all three species (Fig. 1). Consequently, tests for strength of adhesive joint and coating were performed with wood specimens treated at this condition. Mechanical tests showed that shear strength of S. contorta plywood specimens glued with phenol formaldehyde resin increased by about 30% compared with untreated controls (Fig. 2a). Excellent adhesion between wood and phenol formaldehyde resin as indicated by wood fibers adhering on both sides of sheared specimens was observed indicating failure occurred within the wood structure. Similarly, S. contorta wood blocks glued with urea formaldehyde resin resulted in about 40% increase in shear strength (Fig. 2b). No significant improvement or decrease in shear strength in both plywood and wood laminate were observed for G. arborea and A. mangium. The increased shear strength associated with S. contorta may be correlated with surface modification following plasma treatment. Atomic force microscopic (AFM) images of untreated and
plasma treated materials revealed differences in surface topography. AFM phase images of untreated materials showed nodulelike deposits, believed to be amorphous hemicelluloses and extractives, dispersed on the surface of the wood (Figs. 3a, 4a, and 5a). However, surface of plasma treated S. contorta showed that the primary and portion of the secondary cell wall were degraded or etched resulting in the exposure of the S2 layer (Fig. 3b). The S2 layer of woody plant cell wall is characterized by cellulosic fibrils oriented at an acute angle [35]. These fibrils are bundled together to form fibers responsible for the stiff and strong skeletal framework of wood. The exposure of the S2 layer could explain the improved work of adhesion observed in this study. The exposed cellulosic fibrils would provide more polar functional groups (adhesion sites) for enhanced bonding with the resin.
M.N. Acda et al. / International Journal of Adhesion & Adhesives 32 (2012) 70–75
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Fig. 2. Shear strength of untreated and plasma treated S. contorta, G. arborea and A. mangium specimens: (a) 3-ply plywood and (b) 2-ply wood laminate.
Surface characterization of plasma treated S. contorta, G. arborea and A. mangium reported by the authors in a separate paper using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed higher absorbance at wavelengths associated with hydroxyl (OH) and carbonyl (CO) functional groups [26]. Evidently, exposure to plasma conditions used in this study resulted in changes in surface characteristics that could affect reactivity of wood species investigated to polar adhesives and coatings. Similar enhanced adhesion effect was observed by Uehara and Jodai [12] with Dipterocarpus grandiflorus wood bonded with urea– formaldehyde adhesive following plasma treatment. Sakata et al. [13] reported a rapid increase in joint strength using a mixed urea– formaldehyde resin and polyvinyl acetate emulsion after mild plasma treatment. No change in shear strength of adhesive joint was observed for G. arborea and A. mangium following plasma surface treatment. This result is opposite to the higher shear strength observed with S. contorta. AFM topographic images for G. arborea and A. mangium showed the presence of surface deposits larger and thicker ( 1.0 mm) than those observed with untreated materials (Figs. 4b and 5b). These deposits could be remnants of cell wall materials or extractives that were broken down by plasma particles during treatment and re-polymerized on the surface. More tests are needed to confirm the cause and mechanism of this hypothesis. However, the presence of these deposits may have created a weak boundary layer that contributed to the reduced strength of adhesive joint for these two species [36]. The presence of a weak boundary layer is believed to have negative effects on bond formation and adhesion strength [37,38].
Fig. 3. AFM surface images of S. contorta: (a) untreated (2.5 mm 2.5 mm) and (b) plasma treated (2 mm 2 mm).
Pull-off strength of cured (14 day) polyurethane coating applied to plasma treated wood specimens resulted in slight improvement in strength properties of S. contorta and A. mangium compared to that of untreated controls (p 40.173–0.560). No improvement in pull off strength was observed with G. arborea despite improvement in work of adhesion (Fig. 6). The reason for the difference in pull off strength of S. contorta, G. arborea and A. mangium could be traced to increased surface polarity and the presence of extractive on the surface of plasma treated wood as described above. However, since improvement are relatively small and with no change in strength for G. arborea, further experiments are necessary to clarify the effects of plasma treatments on performance of polyurethane coatings on wood. Earlier studies reported varied results on the effect of plasma treatment
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M.N. Acda et al. / International Journal of Adhesion & Adhesives 32 (2012) 70–75
Fig. 4. AFM surface images of G. arborea: (a) untreated and (b) plasma treated specimens (2.5 mm 2.5 mm). Fig. 5. AFM images of untreated (a) and plasma treated (b) specimens of A. mangium (2.5 mm 2.5 mm).
on adhesion strength of coating systems. Back and Danielsson [24] reported improved bonding of a water-based acrylate lacquer on wood following corona treatment while no improvement was found when a solvent-based alkyd system was used. Legeay et al. [39] reported increased adhesion of glycerophtalic varnish on wood after pre-treatment with oxygen plasma but observed decreased adhesion with polyurethane varnish. Podgorski et al. [14] and Podgorski and Roux [40], however, indicated no improvement in adhesion properties of solvent or water based alkyd-acrylic on wood after plasma treatment. Cho and Sjoblom [41] also indicated poor adhesion characteristics of a water-borne paint to plasma treated wood samples. In general, dielectric barrier discharge oxygen affected adhesion properties of commercial glues and polyurethane based coating to S. contorta, G. arborea and A. mangium wood specimens. Plasma intensity from 5.8 to 46.5 kW min/m2 resulted in several fold
increase in work of adhesion with no apparent surface defects or change in bulk properties. The increase in work of adhesion was predominantly due to contribution from polar component and could be positively correlated with the development of good adhesion strength with polar wood adhesives and coatings systems. Shear strength of glue joints from plasma treated wood specimens for all three species tested showed varying results. Plywood and wood laminated samples from S. contorta showed slight improvement in shear strength. No improvement or decrease in shear strength was observed for G. arborea and A. mangium. Surface characterization using AFM following plasma treatment indicated that surface topography could influence bond strength of adhesive joint. Small improvement in pull-off-strengths of cured polyurethane coating on plasma treated S. contorta and A. mangium wood specimens were observed. Apparently, effects of plasma treatment vary with wood
M.N. Acda et al. / International Journal of Adhesion & Adhesives 32 (2012) 70–75
Fig. 6. Pull-off strength of cured polyurethane coating on plasma treated and untreated S. contorta, G. arborea and A. mangium wood specimens.
species and process parameters. Further experiments are necessary to clarify the effects of plasma treatments on surface properties and performance of adhesive joint and film.
Acknowledgments The authors wish to thank the Philippine Council for Advanced Science and Technology Research and Development (PCASTRD), Department of Science and Technology and the University of the Philippines for providing financial support for this project; Karel Pabelina and Joel Guhit of the National Institute of Physics, University of the Philippines, Diliman for their assistance during plasma treatment; Juanito M. Jimenez of the Forest Products Research and Development Institute, Department of Science and Technology for assistance in mechanical testing; and Victor G. Gonzales of the Department of Forest Products and Paper Science for sample preparation and testing. References [1] Kaplan SL, Rose PW. Plasma surface treatment of plastics to enhance adhesion. Int J Adhes Adhes 1991;11:109–13. [2] Kogelschatz U. Dielectric-barrier discharges: their history, discharge physics, and industrial applications. Plasma Chem Plasma Process 2003:)1–46. [3] Custodio J, Broughton J, Cruz H, Hutchinson A. A review of adhesion promotion techniques for solid timber substrates. J Adhes 2008;84:502–29. [4] Eliezer S, Eliezer Y. The fourth state of matter: an introduction to plasma science. 2nd ed. Philadelphia: Institute of Physics Publishing; 2002. [5] Lide M, editor. CRC handbook of chemistry and physics. 84th ed. Boca Raton, FL: CRC Press; 1993. [6] Denes AR, Tshabalala MA, Rowell R, Denes F, Young RA. Reduction of weathering degradation of wood through plasma polymer coating. Holzforschung 1999;53:318–26. [7] Setoyama K. Surface modification of wood by plasma treatment and plasma modification. J Photopolym Sci Technol 1996;9:243–50. [8] Magalhaes WLE, Ferraira De Souza M. 1-Butene-cold plasma coating of solid softwood. In: Proceedings of the second wood coatings congress, The Hague, The Netherlands; 2001. [9] Mahlberg WLE, Niemi H, Denes FS, Rowell RM. Effect of oxygen and hexamethyldisiloxane plasma on morphology, wettability and adhesion properties of polypropylene and lignocellulosics. Int J Adhes Adhes 1997;18:283–97. [10] Rehn P, Viol W. Dielectric barrier discharge treatments at atmospheric pressure for wood surface modification. Holz Roh-Werkst 2003;16:145–50. [11] Frihart CH. Wood adhesion and adhesive. In: Rowel RM, editor. Handbook of wood chemistry and wood composites. Boca Raton, FL: CRC Press; 2005. p. 230–8.
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[12] Uehara T, Jodai S. Gluing of wood by corona treatment. Mokuzai Gakkaishi 1987;33:777–84. [13] Sakata I, Morita M, Tsuruta N, Morita K. Activation of wood surface by corona treatment to improve adhesive bonding. J Appl Polym Sci 1993;49:1251–8. [14] Podgorski L, Chevet B, Onic L, Merlin A. Modification of wood wettability by plasma and corona treatments. Int J Adhes Adhes 2000;20:103–11. ¨ ¨ W. Wood surface modifica[15] Rehn P, Wolkenhauer A, Bente M, Forster S, Viol tion in dielectric barrier discharges at atmospheric pressure. Surf Coat Technol 2003;174–175:515–8. ¨ ¨ W. Wood surface [16] Bente M, Avramidis G, Forster S, Rohwer EG, Viol modification in dielectric barrier discharges at atmospheric pressure for creating water repellent characteristics. Holz-als Roh-und Werstoff 2004;62:157–63. [17] Mertens M, Wolkenhauer A, Viol W. UV laser ablation and plasma treatment of wooden surfaces—a comparing investigation. Laser Phys Lett 2006;3:380–4. [18] De Meijer M, Haemers S, Cobben W, Militz M. Surface energy determinations of wood: comparison methods and wood species. Langmuir 2000;16:9352–9. [19] Gindl M, Reiterer A, Sinn G, Stanzl-Tschegg SE. Effects of surface ageing on wettability, surface chemistry and adhesion of wood. Holz-als Roh-und Werstoff 2004;62:273–80. ¨ W. Increased PVA–glue adhesion on particle [20] Wolkenhauer A, Militz H, Viol board and fibre board by plasma treatment. Holz als Roh- und Werkstoff 2008;66:143–5. [21] Furuno T, Uehara T. Histochemical studies of corona-treated wood. Mokuzai Gakkaishi 1990;36:323–31. [22] Jie-Rong C, Xue-Yan W, Tomiji W. Wettability of poly(ethylene terephthalate) film treated with low-temperature plasma and their surface analysis by ESCA. J Appl Polym Sci 1999;72:1327–33. [23] Odraskova´ M, Szalay Z, Ra´hel J, Zahoranova´ A, Cerna´k M. Plasma activation of wood surface by diffuse coplanar surface barrier discharge. In: Proceedings of the 28th international conference on phenomena in ionized gases, Prague, Czech Republic; 2008. [24] Back EL, Danielson S. Oxidative activation of wood and paper surfaces for bonding and for paint adhesion. Nordic Pulp Paper Res J 1987;2:53–62. [25] Denes AR, Young RA. Reduction of weathering degradation of wood through plasma polymer coating. Holzforschung 1999;53:632–40. [26] Acda MN, Devera EE, Cabangon RJ, Pabelina KG, Ramos HJ. Effects of plasma modification on surface properties of wood. Wood Sci Technol, in preparation. ¨ H. Investigation of wood [27] Wolkenhauer A, Avramidis G, Cai Y, Militz H, Viol and timber surface modification by dielectric barrier discharge at atmospheric pressure. Plasma Process Polym 2008;4:470–4. [28] Owens DK, Wendt RC. Estimation of the surface free energy of polymers. J Appl Polym Sci 1969;13:1741–7. [29] Philippine National Standard. Plywood-Specification, PNS 196. Makati, Philippines: Bureau of Products Standards; 2000. [30] ASTM. Strength properties of adhesive bond in shear by compression loading. In: Annual book of ASTM standards, Part 22, Wood, adhesives. Philadelphia, PA: American Society of Testing Materials; 1995. [31] Statgraphics. Statgraphics plus for Windows 4 users manual. Rockville, MD: Manugistics Inc.; 1999. [32] ASTM. Standard test method for pull off strength of coating using portable adhesion testers D 4541. Philadelphia, PA: American Society of Testing Materials; 2002. ¨ W. Sanding vs. [33] Wolkenhauer A, Avramidis G, Hauswald E, Militz H, Viol plasma treatment of aged wood: a comparison with respect to surface energy. Int J Adhes Adhes 2009;29:18–22. [34] Asandulesa M, Topola I, Dumitrascu N. Effect of helium DBD plasma treatment on the surface of wood samples. Holzforschung 2010;64:223–7. [35] Panshin AJ, de Zeeuw C. Texbook of wood technology, vol. 1. New York: McGraw-Hill Book Company; 1970. p. 75–6. [36] Frihart CH, Hunt CG. Adhesives with wood materials—bond formation and performance. In: Wood handbook—wood as an engineering material. General technical report FPL-GTR-190. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory; 2010. p. 1–24. [37] Gardner DJ. Adhesion mechanisms of durable wood adhesive bonds. In: Stokke DD, Groom LH, editors. Characterization of the cellulosic cell wall. Ames, IA: Blackwell Publishing; 2005. p. 254–65. [38] Stehr M, Johannson I. Weak boundary layers on wood surfaces. J Adhes Sci Technol 2000;14:1211–24. [39] Legeay G, Epaillard F, Brosse JC. Surface modification of natural and synthetic polymers by cold plasmas. In: Proceedings of the second annual international conference on plasma chemistry and technology; 1984. p. 29–39. [40] Podgorski L, Roux ML. Wood modification to improve the durability of coatings. Surf Coat Int 1999;82:590–6. [41] Cho DL, Sjoblom E. Plasma treatment of wood. Polym Mater: Sci Eng 1990;62:48–9.
Contents lists available at SciVerse ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effects of plasma modification on adhesion properties of wood Menandro N. Acda a,n, Edgar E. Devera a, Rico J. Cabangon b, Henry J. Ramos c a b c
Department of Forest Products and Paper Science, University of the Philippines Los Banos College, Laguna 4031, Phillipines Forest Products Research and Development Institute, Department of Science and Technology, Laguna 4031, Phillipines National Institute of Physics, University of the Philippines Diliman, Diliman, Quezon City 1101, Phillipines
a r t i c l e i n f o
a b s t r a c t
Article history: Accepted 10 October 2011 Available online 26 October 2011
The study investigated the use of dielectric barrier discharge for surface modification to improve adhesion properties of Shorea contorta (white lauan), Gmelina arborea (yemane) and Acacia mangium. Wood specimens were exposed to oxygen plasma at intensity levels ranging from 5.8 to 46.5 kW min/ m2 to improve adhesion of phenol formaldehyde, urea formaldehyde resins and polyurethane coating. Work of adhesion was calculated based on contact angle measurements to determine thermodynamic changes on plasma modified wood. Surface characteristic was evaluated using atomic force microscopy (AFM). Results of the study showed that plasma modification resulted in significant improvement in work of adhesion for the three wood species investigated. Mechanical tests of plywood and wood laminates using plasma treated S. contorta glued with phenol or urea formaldehyde resins indicated improvement in shear strength of adhesive joints. No improvement or decrease in shear strengths were observed for plasma treated G. arborea and A. mangium. Pull off strengths of polyurethane coating on plasma treated S. contorta and A. mangium specimens showed slight improvement in strength of coated film. Effects of plasma treatment on adhesion properties of wood appear to be species specific and vary with process parameters. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Plasma Surface modification Wood Atomic force microscopy
1. Introduction In recent years, cold plasmas have been used to modify surface properties of wood and paper to improve adhesion of glues or enhance deposition of thin layer of polymers to the surface of metals, glasses and plastics [1–3]. Cold plasma is a mixture excited particles (ions, electrons, atoms, radicals, etc.) with very low degrees of ionization and little penetrating energy [4]. However, plasma particles have sufficient energy levels to break chemical bonds on the surface of organic and inorganic substrates [5,6]. The broken chemical bonds are thermodynamically unstable and combine readily with gas fragment to molecularly re-engineer the surface of the material [7]. The modification (surface cleaning, ablation and degradation, cross-linking, surface oxidation and polymerization), however, is limited to a few molecular layers and does not affect bulk properties. The effects may occur concurrently or one may predominate depending on the processing conditions and the design of the reaction chamber. Plasma treatments have been successfully applied in a wide range of surface modification and enhanced chemical vapor deposition [1–3]. This approach is also attracting research interest for surface
n
Corresponding author. Tel.: þ49 536 3432; fax: þ 49 536 3206. E-mail address: [email protected] (M.N. Acda).
0143-7496/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.10.003
modification to improve adhesion and performance of wood coatings. Studies showed that plasma can be used to improve the wettability of wood to liquid [6,8–10]. Wettability assists in establishing an extensive and molecular scale contact with the wood surface and critical to the development of strong adhesion at the adhesive–wood interface [11]. Cold plasmas have been used to improve the wetting and adhesion properties of wood [12–17] and wood composites [18–20]. It is believed that plasma modification results in increased surface polarity brought about by oxidation reaction leading to the formation of hydroxyl, carboxyl, aldehyde and other polar functional groups [21–23]. The increased surface polarity improves wettability and penetration behavior (hydrophilicity) of wood to liquid [6,8–10]. Studies on the effects of plasma treatments to improve strength of wood finishes and coating systems, however, are limited. Atmospheric corona discharge, a variation of plasma treatment, has been used to increase the surface energy of wood surfaces by oxidative activation [24]. Improved bonding of a water-based acrylate lacquer was found following corona treatment of wood surface but no improvement when a solvent-based alkyd system was used. Denes and Young [25] coated wood with polydimethylsiloxane (PDMSO) containing various UV stabilizers, absorbers or reflectors, and polymerized the coating using an oxygen plasma. The use of such coatings significantly improved the weathering resistance of the coated
M.N. Acda et al. / International Journal of Adhesion & Adhesives 32 (2012) 70–75
wood. Shear strength of hardwood and softwood wood specimens coated with lacquer adhesive improved with corona treatment while delamination of coatings on wetted bonded samples was reduced significantly [15]. Plasma surface treatment shows tremendous potential for improving adhesion of glues and coatings to wood. Consequently, new material combinations, elimination of primers and cost savings are conceivable benefits [3]. The present paper reports on the use of plasma modification to improve adhesion properties of three species of tropical hardwoods, viz., Shorea contorta Vidal, Gmelina arborea Roxb. and Acacia mangium Willd. commonly used in the Philippines and Southeast Asia as raw material for furniture and wood based panel products.
2. Materials and methods 2.1. Wood specimen
71
calculated using the Dupre equation W A ¼ gS þ gL gSL
ð1Þ
where gS is the surface free energy of solid, gL is the surface tension of the liquid and gSL is the interfacial energy between solid and liquid. For optimum adhesion, WA must be maximized, which occurs when the interfacial tension (gSL) becomes zero [20,27]. The interfacial tension between solid and liquid is given by the geometric mean equation [28] qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi ð2Þ gSL ¼ gS þ gL 2 gDS gDL 2 gPS gPL P D where gD L and gL are the dispersive and polar part of liquid and gS and gPS are dispersive and polar part of solid (i.e. surface free energy of wood species). Since many paints and wood coatings are water based, work of adhesion was determined in terms of dispersive and polar components: P WA ¼ WD A þW A
ð3Þ
and using Eqs. (1)–(3) qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi D D WD and W PA ¼ 2 gPL gPS A ¼ 2 gL gS
ð4Þ
3
Kiln dried S. contorta (white lauan, density 430 kg/m ) lumber obtained from local furniture manufacturer in Cebu province, G. arborea (yemane, density 410 kg/m3) and A. mangium (density 470 kg/m3) from the Forest Products Research Development Institute, Philippines were used in this study. These species are grown intensively in industrial plantations and commonly used throughout the Philippines and Southeast Asia for furniture and panel manufacture. Veneer specimens (2 mm 100 mm 100 mm) and wood blocks (10 mm 50 mm 50 mm) were cut from the heartwood of straight grained and defect-free boards and conditioned for several weeks at 24 1C and 65% relative humidity to bring the moisture content to 8% prior to plasma modification.
2.2. Plasma treatment Plasma surface modification to improve adhesion property of S. contorta, G. arborea and A. mangium wood specimens was performed using a dielectric barrier discharge (DBD) developed at the National Institute of Physics, University of the Philippines, Diliman, Philippines and described in an earlier paper [26]. Briefly, high purity oxygen was admitted between the electrodes at reduced pressure (10 Pa). Mass flow, temperature, pressure and electric potential were closely monitored and recorded using data acquisition instruments. Surfaces of veneers and wood blocks to be coated with glue were exposed to dielectric barrier discharge ranging in intensities from 5.8 to 46.5 kW min/m2 at 3 mm gap between electrodes to determine its effect on adhesion properties. The DBD chamber was thoroughly cleaned with ethanol after each treatment to remove deposits and contaminants from previous treatment. Untreated wood specimens were used as control. Surface characterization of plasma treated and untreated specimens were performed using an atomic force microscope (Solver PRO, NT-MDT, Netherlands) running on a Nova 1.0.26C software. All measurements were performed at atmospheric pressure and room temperature in non contact tapping mode (scan size 1.0– 2.5 mm2, scan rate 0.3–1 Hz with a silicon cantilever tip, 256 pixels by 256 pixels resolution). Topographic and phase contrast images of plasma treated specimens were obtained and compared with untreated controls.
2.3. Work of adhesion The work of adhesion with reference to water was used to assess the effect of plasma treatment on adhesion properties of all three wood species investigated. The work of adhesion (WA) was
The surface free energies (gS ) of S. contorta, G. arborea and A. mangium wood specimens were calculated based on the Owens, Wendt, Rabel and Kaeble method and reported in an earlier paper [26]. Water (gL ) has a known polar and dispersive components (gP ¼51 mN/m, gD ¼21.8 mN/m). 2.4. Adhesive bond strength Three ply plywood and 2-ply wood laminates were constructed using specimens from treatment combination that showed optimum improvement in surface energy and work of adhesion. Plywood was constructed using a commercial hot setting phenol formaldehyde resin (Phenoress, 45% resin solids, RI Chemical Corp., Pasig, Philippines) following gluing and pressing conditions recommended by the manufacturer (glue spread 130 g/m2, specific pressure 10 kg/cm2, press temperature 130 1C, total pres time 3.5 min, assembly time 5 min). Glue was applied on plasma modified surfaces (i.e. underside of face and back veneers; and both sides of core veneer). Wood blocks were laminated (185 g/m2 spread rate, 10 kg/cm2 specific pressure and 24 h press time) using a cold setting urea formaldehyde resin (Consolidated Adhesives, Inc.) commonly used for wooden parquet and flooring in the Philippines. Standard plywood and wood block shear specimens were prepared and used to evaluate the effects of plasma modification on strength properties of adhesive joints in compression and tensile loading. All specimens were conditioned for 6 weeks at 24 1C and 65% relative humidity prior to adhesive bond testing. Bond strength of plywood specimens was measured using the cyclic-boil, hot–cold soaking test of the Philippine National Standard 196 [29]. Dry shear strength of wood block laminates was measured following the American Society for Testing and Material 906 [30]. Testing was carried out using a universal testing machine at crosshead speed of 0.5 mm/min. Shear force required to break the adhesive bond to failure was recorded by data acquisition software. Grain direction was ensured parallel to the shearing stress for all specimen tested. Four replicate specimens were tested for each treatment combination. Data for shear strength and plasma intensity were fitted in a completely randomized design and evaluated using analysis of variance (ANOVA) using Statgraphics Plus for Windows 4 software [31]. Treatment means were separated by Tukey’s Honest Significance Difference (HSD) test (a ¼0.05).
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2.5. Coating and pull-off test Plasma treated and untreated wood specimens were spray coated two times with a two component polyurethane coating system (Topcoat 20-AAA varnish and polyurethane catalyst-AAA, Century Chemical Corporation, Quezon City, Philippines) within 6 h following plasma treatment. Polyurethane coatings are commonly used for wood finishes and parquet floors in the Philippines. A final coating was applied after a light sanding (320 grit). The coated specimens were stored in a conditioning room at 24 1C and 65% relative humidity for 2 weeks prior to mechanical testing. The adhesion of the polyurethane films on both treated and untreated specimens was evaluated using a modified pull-off test following ASTM D 4541 [32]. A universal testing machine (5 kN capacity) fitted with a fabricated jig consisting of a 20-mmdiameter steel dolly and sample holder was used in this study. The dolly was glued on the coated wood surface with an epoxy resin. After 6 h of curing at 24 1C and 65% RH, the perimeters of the glued dollies were carefully cleaned to prevent propagation of failures out of the tested area. Constant pull of the testing machine was applied at 5 mm/min until separation of the dolly from the plasma treated and untreated specimens. The maximum normal pull strength at rupture was recorded using data acquisition software. Pull off strength data was analyzed for each species and compared with untreated controls as described above.
3. Results and discussion 3.1. Effects on work of adhesion Plasma surface modification of S. contorta, G. arborea and A. mangium using dielectric barrier discharge at intensities used in this study resulted in 2–3 times increase in work of adhesion compared with untreated materials (Fig. 1). No apparent surface defects or negative effects on bulk properties were observed following plasma treatment. Improvement in the work of adhesion was predominantly caused by significant increase in the polar component (p40.001) for all three species. Similar increase in work of adhesion were also reported using air or helium DBD treatment of oak, beech, spruce and Oregon pine [33,34]. Since most coating systems for wood are water based, plasma treatment could have potential beneficial effects in improving adhesion properties of S. contorta, G. arborea and A. mangium to polar coating systems. 3.2. Effects of plasma treatment on strength of adhesive bond
Fig. 1. Work of adhesion for untreated and plasma treated S. contorta, G. arborea and A. mangium wood specimens.
2
Plasma intensity at 5.8 kW min/m was the lowest level that resulted in significant improvement in work of adhesion for all three species (Fig. 1). Consequently, tests for strength of adhesive joint and coating were performed with wood specimens treated at this condition. Mechanical tests showed that shear strength of S. contorta plywood specimens glued with phenol formaldehyde resin increased by about 30% compared with untreated controls (Fig. 2a). Excellent adhesion between wood and phenol formaldehyde resin as indicated by wood fibers adhering on both sides of sheared specimens was observed indicating failure occurred within the wood structure. Similarly, S. contorta wood blocks glued with urea formaldehyde resin resulted in about 40% increase in shear strength (Fig. 2b). No significant improvement or decrease in shear strength in both plywood and wood laminate were observed for G. arborea and A. mangium. The increased shear strength associated with S. contorta may be correlated with surface modification following plasma treatment. Atomic force microscopic (AFM) images of untreated and
plasma treated materials revealed differences in surface topography. AFM phase images of untreated materials showed nodulelike deposits, believed to be amorphous hemicelluloses and extractives, dispersed on the surface of the wood (Figs. 3a, 4a, and 5a). However, surface of plasma treated S. contorta showed that the primary and portion of the secondary cell wall were degraded or etched resulting in the exposure of the S2 layer (Fig. 3b). The S2 layer of woody plant cell wall is characterized by cellulosic fibrils oriented at an acute angle [35]. These fibrils are bundled together to form fibers responsible for the stiff and strong skeletal framework of wood. The exposure of the S2 layer could explain the improved work of adhesion observed in this study. The exposed cellulosic fibrils would provide more polar functional groups (adhesion sites) for enhanced bonding with the resin.
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Fig. 2. Shear strength of untreated and plasma treated S. contorta, G. arborea and A. mangium specimens: (a) 3-ply plywood and (b) 2-ply wood laminate.
Surface characterization of plasma treated S. contorta, G. arborea and A. mangium reported by the authors in a separate paper using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed higher absorbance at wavelengths associated with hydroxyl (OH) and carbonyl (CO) functional groups [26]. Evidently, exposure to plasma conditions used in this study resulted in changes in surface characteristics that could affect reactivity of wood species investigated to polar adhesives and coatings. Similar enhanced adhesion effect was observed by Uehara and Jodai [12] with Dipterocarpus grandiflorus wood bonded with urea– formaldehyde adhesive following plasma treatment. Sakata et al. [13] reported a rapid increase in joint strength using a mixed urea– formaldehyde resin and polyvinyl acetate emulsion after mild plasma treatment. No change in shear strength of adhesive joint was observed for G. arborea and A. mangium following plasma surface treatment. This result is opposite to the higher shear strength observed with S. contorta. AFM topographic images for G. arborea and A. mangium showed the presence of surface deposits larger and thicker ( 1.0 mm) than those observed with untreated materials (Figs. 4b and 5b). These deposits could be remnants of cell wall materials or extractives that were broken down by plasma particles during treatment and re-polymerized on the surface. More tests are needed to confirm the cause and mechanism of this hypothesis. However, the presence of these deposits may have created a weak boundary layer that contributed to the reduced strength of adhesive joint for these two species [36]. The presence of a weak boundary layer is believed to have negative effects on bond formation and adhesion strength [37,38].
Fig. 3. AFM surface images of S. contorta: (a) untreated (2.5 mm 2.5 mm) and (b) plasma treated (2 mm 2 mm).
Pull-off strength of cured (14 day) polyurethane coating applied to plasma treated wood specimens resulted in slight improvement in strength properties of S. contorta and A. mangium compared to that of untreated controls (p 40.173–0.560). No improvement in pull off strength was observed with G. arborea despite improvement in work of adhesion (Fig. 6). The reason for the difference in pull off strength of S. contorta, G. arborea and A. mangium could be traced to increased surface polarity and the presence of extractive on the surface of plasma treated wood as described above. However, since improvement are relatively small and with no change in strength for G. arborea, further experiments are necessary to clarify the effects of plasma treatments on performance of polyurethane coatings on wood. Earlier studies reported varied results on the effect of plasma treatment
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Fig. 4. AFM surface images of G. arborea: (a) untreated and (b) plasma treated specimens (2.5 mm 2.5 mm). Fig. 5. AFM images of untreated (a) and plasma treated (b) specimens of A. mangium (2.5 mm 2.5 mm).
on adhesion strength of coating systems. Back and Danielsson [24] reported improved bonding of a water-based acrylate lacquer on wood following corona treatment while no improvement was found when a solvent-based alkyd system was used. Legeay et al. [39] reported increased adhesion of glycerophtalic varnish on wood after pre-treatment with oxygen plasma but observed decreased adhesion with polyurethane varnish. Podgorski et al. [14] and Podgorski and Roux [40], however, indicated no improvement in adhesion properties of solvent or water based alkyd-acrylic on wood after plasma treatment. Cho and Sjoblom [41] also indicated poor adhesion characteristics of a water-borne paint to plasma treated wood samples. In general, dielectric barrier discharge oxygen affected adhesion properties of commercial glues and polyurethane based coating to S. contorta, G. arborea and A. mangium wood specimens. Plasma intensity from 5.8 to 46.5 kW min/m2 resulted in several fold
increase in work of adhesion with no apparent surface defects or change in bulk properties. The increase in work of adhesion was predominantly due to contribution from polar component and could be positively correlated with the development of good adhesion strength with polar wood adhesives and coatings systems. Shear strength of glue joints from plasma treated wood specimens for all three species tested showed varying results. Plywood and wood laminated samples from S. contorta showed slight improvement in shear strength. No improvement or decrease in shear strength was observed for G. arborea and A. mangium. Surface characterization using AFM following plasma treatment indicated that surface topography could influence bond strength of adhesive joint. Small improvement in pull-off-strengths of cured polyurethane coating on plasma treated S. contorta and A. mangium wood specimens were observed. Apparently, effects of plasma treatment vary with wood
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Fig. 6. Pull-off strength of cured polyurethane coating on plasma treated and untreated S. contorta, G. arborea and A. mangium wood specimens.
species and process parameters. Further experiments are necessary to clarify the effects of plasma treatments on surface properties and performance of adhesive joint and film.
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